Our results indicate that the glutamatergic system is involved in the mechanism of action of rapid antidepressant response. In addition, this system may be a feasible target for developing treatments that have rapid and robust efficacy in individuals who have treatment-resistant depression and suicidal thoughts. We found that the glutamatergic modulator ketamine resulted in rapid, robust and relatively sustained antidepressant, antisuicidal, and antianhedonic effects. Response with ketamine occurred within 2 hours and lasted approximately 1 week. Comparable response rates with standard of care treatments occur at 6-8 weeks instead of hours. Study: (Biomarkers of rapid response in major depressive disorder): protocols 04-M-0222 (NCT00088699) and 17-M-0060 (NCT03065335). OBJECTIVE: To identify the neural correlates of rapid antidepressant response to the NMDA antagonist ketamine in subjects with major depressive disorder. We found robust and rapid antidepressant effects resulted from a single intravenous dose of an N-methyl-D-aspartate antagonist (ketamine); onset occurred within 2 hours post-infusion and continued to remain significant for 1 week. Aims are 1)to examine the antisuicidal effects of ketamine, and 2)to examine correlates of antidepressant response to ketamine in both major depressive disorder and bipolar disorder and include these data/outcome measures: clinical (e.g., family history), imaging (magnetic resonance imaging/spectroscopy), electrophysiological (magnetoencephalography MEG, electroencephalography EEG), neuropsychological, and biochemical (e.g., genetics, microRNA, BDNF, metabolomics), 3) To demonstrate more robust neuropharmacodynamic effects measured by neuropharmacodynamic imaging (fMRI+EEG and MEG) of ketamine 0.5 mg/kg as compared to placebo administered over 40 minutes. Secondary Outcome Measures: To determine if increases in synaptic plasticity, using electrophysiological measures in response to TMS and in association with sleep (i.e., slow wave sleep EEG activity) are associated with better antidepressant response to 0.5 mg/kg Time Frame: baseline and post-drug To demonstrate enhanced efficacy, as measured by the MADRS, of IV ketamine 0.5 mg/kg in participants with major depressive disorder (MDD) using a psychophysiological technique (i.e. NPU-threat test). Time Frame: baseline and post-drug To identify baseline peripheral measures associated with response to the administration of ketamine 0.5 mg/kg, as potential biomarkers of acute (24 hour) treatment response. Time Frame: baseline and post-drug Results in the past year: 1. Multimodal imaging reveals a complex pattern of dysfunction in corticolimbic pathways in major depressive disorder: While numerous studies have investigated the neurobiological correlates of MDD, most have used only a single neuroimaging modality. In particular, diffusion tensor imaging (DTI) studies have failed to yield uniform results. In this context, examining key tracts and using information from multiple neuroimaging modalities may better characterize potential abnormalities in the MDD brain. This study analyzed data from 30 participants with MDD and 26 healthy participants who underwent DTI, magnetic resonance spectroscopy (MRS), resting-state functional magnetic resonance imaging (fMRI), and magnetoencephalography (MEG). Tracts connecting the subgenual anterior cingulate cortex (sgACC) and the left and right amygdala, as well as connections to the left and right hippocampus and thalamus, were examined as target areas. Reduced fractional anisotropy (FA) was observed in the studied tracts. Significant differences in the correlation between medial prefrontal glutamate concentrations and FA were also observed between MDD and healthy participants along tracts connecting the sgACC and right amygdala; healthy participants exhibited a strong correlation but MDD participants showed no such relationship. In the same tract, a correlation was observed between FA and subsequent antidepressant response to ketamine infusion in MDD participants. This study is the first to combine MRS, DTI, fMRI, and MEG data to obtain multimodal indices of MDD and antidepressant response and may lay the foundation for similar future analyses. 2. Effects of ketamine on brain activity during emotional processing: differential findings in depressed versus healthy control participants: Experimental tasks involving emotional processing can be used during functional magnetic resonance imaging scanning to investigate ketamine's effects on brain function in MDD. A total of 33 individuals with treatment-resistant MDD and 24 healthy control participants (HCs) took part in this double-blind, placebo-controlled crossover study. Participants received ketamine and placebo infusions 2 weeks apart, and functional magnetic resonance imaging scans were conducted at baseline and 2 days after each infusion. Blood oxygen level-dependent signal was measured during an emotional processing task. A group-by-drug interaction was observed in several brain regions, including a right frontal cluster extending into the anterior cingulate cortex and insula. Participants with MDD had greater activity than HCs after placebo infusion but showed lower activity after ketamine infusion, which was similar to the activity in HCs after placebo. The main results indicate that ketamine had differential effects on brain activity in participants with MDD versus HCs. The pattern of activation in participants with MDD after ketamine infusion resembled the activation in HCs after placebo infusion, suggesting a normalization of function during emotional processing. The findings contribute to a better understanding of ketamine's actions in the brain. 3. Ketamine has distinct electrophysiological and behavioral effects in depressed and healthy subjects. Ketamine's mechanism of action was assessed using gamma power from magnetoencephalography (MEG) as a proxy measure for homeostatic balance in 35 unmedicated subjects with major depressive disorder (MDD) and 25 healthy controls enrolled in a double-blind, placebo-controlled, randomized cross-over trial of 0.5mg/kg ketamine. MDD subjects showed significant improvements in depressive symptoms, and healthy control subjects exhibited modest but significant increases in depressive symptoms for up to 1 day after ketamine administration. Both groups showed increased resting gamma power following ketamine. In MDD subjects, gamma power was not associated with the magnitude of the antidepressant effect. However, baseline gamma power was found to moderate the relationship between post-ketamine gamma power and antidepressant response; specifically, higher post-ketamine gamma power was associated with better response in MDD subjects with lower baseline gamma, with an inverted relationship in MDD subjects with higher baseline gamma. This relationship was observed in multiple regions involved in networks hypothesized to be involved in the pathophysiology of MDD. This finding suggests biological subtypes based on the direction of homeostatic dysregulation and has important implications for inferring ketamine's mechanism of action from studies of healthy controls alone. 4. Plasma metabolomic profiling of a ketamine and placebo crossover trial of major depressive disorder and healthy control subjects. The metabolomic profile of these subjects was characterized at multiple time points in MDD and healthy controls, treatment and placebo in both groups and the corresponding response to ketamine treatment. Ketamine response resulted in more pronounced effects in the kynurenine pathway and the arginine pathway at 4 h post-infusion, where a larger decrease in circulating kynurenine levels and a larger increase in the bioavailability of arginine were observed in responders to ketamine treatment.